Metal-poly integrated capacitor structure

ABSTRACT

A metal-poly integrated capacitor structure that may be used in a charge pump circuit of a non-volatile memory. In one embodiment, the capacitor comprises a poly silicon layer, a first metal layer and a second metal layer. The first metal layer is positioned between the poly silicon layer and the second metal layer. The first metal layer has a first terminal and a second terminal. The first terminal is electrically isolated from the second terminal.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/228,823, titled “METAL-POLY INTEGRATED CAPACITOR STRUCTURE,” filedAug. 27, 2002, now U.S. Pat. No. 6,897,511, which application isassigned to the assignee of the present invention and the entirecontents of which are incorporated herein by reference, and whichapplication claims priority to Italian Patent Application Serial No.RM2001A000517, filed Aug. 29, 2001, of the same title.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to capacitors and in particularthe present invention relates to a metal-poly integrated capacitorstructure that may be used in a charge pump circuit of a non-volatilememory.

BACKGROUND OF THE INVENTION

A flash memory is a type of non-volatile memory. That is, a flash memoryis a type of memory that retains stored data without a periodic refreshof electricity. An important feature of a flash memory is that it can beerased in blocks instead of one byte at a time. Each erasable block ofmemory comprises a plurality of non-volatile memory cells (cells)arranged in rows and columns. Each cell is coupled to a word line, bitline and source line. In particular, a word line is coupled to a controlgate of each cell in a row, a bit line is coupled to a drain of eachcell in a column and the source line is coupled to a source of each cellin an erasable block. The cells are programmed, read and erased bymanipulating the voltages on the word lines, bit lines and source lines.

The voltage level needed to program or erase a non-volatile memory cellcan be has high as 12 volts or more. Since an external Vcc power supplyto a flash memory is typically 1.8 volts or lower, internal charge pumpsare used in the flash memory to provide the required voltage. Theinternal charge pump is used to boost the external Vcc power supplyvoltage to a required voltage. Traditionally, charge pumps do notsupport high current loads, hence the resistive load of the charge pumpmust be kept at a minimum. Accordingly, charge pump circuits typicallyincorporate a capacitive voltage divider instead of a resistance voltagedivider. For reliable operation of a flash memory, a well regulatedcharge pump is required. However, typical capacitors used in a voltagedivider circuit of an integrated flash memory tend to not be as stableor precise as desired which have a negative effect on the reliability ofthe charge pump circuit.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora flash memory architecture having a charge pump circuit thatincorporates a voltage divider with stable and precise capacitors.

SUMMARY OF THE INVENTION

The above-mentioned problems with memory devices and other problems areaddressed by the present invention and will be understood by reading andstudying the following specification.

In one embodiment, a capacitor is disclosed. The capacitor comprises apoly silicon layer, a second metal layer and a first metal layer. Thefirst metal layer is positioned between the poly silicon layer and thesecond metal layer. The first metal layer has a first terminal and asecond terminal. Moreover, the first terminal is electrically isolatedfrom the second terminal.

In another embodiment, a capacitor is formed in an integrated circuitthat comprises a first metal layer, a second metal layer and a polysilicon layer. The first metal layer has a first terminal and a secondterminal. The first and second terminals each have a base strip and aplurality of side strips that extend from their respective base strip.The side strips of the first terminal are positioned in between the sidestrips of the second terminal so that the first and second side stripsare alternately positioned between the first and second bases of therespective first and second terminals. In addition, the first terminalis electrically isolated from the second terminal. Moreover, the firstmetal layer is positioned between the poly silicon layer and the secondmetal layer.

In another embodiment, a voltage divider comprises a first and secondcapacitor. The first capacitor has a second terminal that is selectivelycoupled to a voltage supply. The second capacitor has a first terminalcoupled to a first terminal of the first capacitor. The second capacitorfurther has a second terminal coupled to ground. The first and secondcapacitors include a first metal layer forming the first and secondterminals. The first and second terminals each have a base strip and aplurality of side strips that extend from the respective base strip. Inaddition, the side strips of the first terminal are positioned inbetween the side strips of the second terminal so that the first andsecond side strips are alternately positioned between the first andsecond bases of the respective first and second terminals. Moreover, thefirst terminal is electrically isolated from the second terminal. Thefirst and second capacitors also include a second metal layer and a polysilicon layer. The first metal layer is positioned between the polysilicon layer and the second metal layer.

In another embodiment, a charge pump circuit comprises a charge pump, afirst and second capacitor, a first, second and third transistor, adifferential amplifier and a AND gate. The charge pump provides anoutput voltage signal. The first capacitor has a first and secondterminal. The second capacitor also has a first and second terminal. Thefirst terminal of the second capacitor is coupled to the first terminalof the first capacitor. Each of the first and second capacitors include,a poly silicon layer, a second metal layer, and a first metal layer. Thefirst metal layer is positioned between the poly silicon layer and thesecond metal layer. The first metal layer has a first terminal and asecond terminal. The first terminal is electrically isolated from thesecond terminal. The first transistor is coupled to selectively couplethe second terminal of the first capacitor to the output signal of thecharge pump. The second transistor is coupled between the secondterminal of the first capacitor and ground. The third transistor iscoupled between the first terminals of the first and second transistorsand ground. Gates of the first, second and third transistors are coupledto a reset signal. The differential amplifier has a first input coupledto the first terminals of the first and second capacitors and a secondinput coupled to a voltage reference. The AND gate has a first inputcoupled to an output of the differential amplifier and a second inputcoupled to a clock pulse. An output of the AND gate is coupled to aninput of the charge pump.

In another embodiment, a non-volatile memory device comprises a memoryarray, control circuitry, an address register, an input/output bufferand a charge pump. The memory array has a plurality of non-volatilememory cells to store data. The control circuitry is used to controlmemory operations to the memory array. The address register is used toselectively couple address requests to the memory array. Theinput/output buffer is used to smooth out data flowing to and from thememory array. The charge pump circuit is used to boost voltage levelsduring select memory operations. The charge pump circuit has capacitors.Each capacitor includes a poly silicon layer, a second metal layer and afirst metal layer. The first metal layer is positioned between the polysilicon layer and the second metal layer. The first metal layer has afirst terminal and a second terminal. In addition, the first terminal iselectrically isolated from the second terminal.

In another embodiment, a flash memory system comprises a processor, amemory array, control circuitry, an address register, an input/outputbuffer and a charge pump. The processor is used to provide externaldata. The memory array is used to store the external data. The controlcircuitry is used to control memory operations to the memory array. Thecontrol circuitry coupled to receive control commands from theprocessor. The address register is used to selectively couple addressrequests to the memory array. The input/output buffer is used to smoothout data flowing to and from the memory array. The charge pump circuitis used to boost voltage levels during select memory operations. Thecharge pump circuit includes a first and second capacitor. Eachcapacitor includes a first metal layer, a second metal layer and a polysilicon layer. The first metal layer forms first and second terminals.The first and second terminals each have a base strip and a plurality ofside strips that extend from the respective base strip. The side stripsof the first terminal are positioned in between the side strips of thesecond terminal so that the first and second side strips are alternatelypositioned between the first and second bases of the respective firstand second terminals. The first metal layer is positioned between thepoly silicon layer and the second metal layer.

A method of forming a capacitor comprising, forming a poly siliconlayer, forming a first metal layer over the poly silicon layer,fabricating the first metal layer to form a first terminal and a secondterminal, wherein the first terminal is electrically isolated from thesecond terminal and forming a second metal layer over the first metallayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a charge pump circuit of the prior art;

FIG. 2 is a timing diagram of a reset signal and enable signal for acharge pump circuit of the prior art;

FIG. 3 is a cross-sectional view of a capacitor formed in an integratedcircuit of the prior art;

FIG. 3A is a schematic diagram of a capacitor of the prior art;

FIG. 4 is a graph illustrating the non-linear characteristics of acapacitor of the prior art;

FIG. 5 is cross-sectional top view of the first metal layer of oneembodiment of the present invention;

FIG. 5A is a cross sectional side view of one embodiment of the presentinvention;

FIG. 6 is a schematic diagram of a charge pump circuit of one embodimentof the present invention; and

FIG. 7 is a block diagram of a flash memory of one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of present embodiments, referenceis made to the accompanying drawings that form a part hereof, and inwhich is shown by way of illustration specific embodiments in which theinventions may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the claims and the equivalents thereof.

The present invention provides stable and precise capacitors in a chargepump circuit to produce a stable voltage supply. Before a detaileddescription of the present invention is given, further background isfirst provided to aid the reader in under standing the presentinvention. Referring to FIG. 1 a charge pump circuit 100 of the priorart is illustrated. The charge pump circuit 100 of FIG. 1 is used tocontrol output voltage Vpp of charge pump 102. The charge pump circuit100 is based on a simple negative feedback circuit using a capacitivevoltage divider (instead of a resistive voltage divider in order toreduce the current load of the pump.). Capacitors 104 (C1) and 106 (C2)make up a capacitive voltage divider 118. N-channel MOS transistors 108(M1), 110 (M2) and 112 (M3) provide a reset of the voltage divider. Adifferential amplifier 114 is also provided. The voltage at node B isVb=Vpp*C1/(C1+C2). Vref is a reference voltage generated on a chip by adedicated circuit (not shown). Enable is the output signal of thedifferential amplifier 114. An AND gate 116 is provided to selectivelypass a clock signal. The clock signal is needed by the charge pump 102to generate Vpp from Vcc (the chip power supply).

At power up, a short reset signal is generated by a circuit (not shown)that completely discharges capacitors 104 (C1) and 106 (C2). The resetis needed to insure the correct operation of the capacitive voltagedivider 118. Otherwise, 104 (C1) and 106 (C2) would be charged to anincorrect voltage at the onset of the operation. The reset isaccomplished by turning off transistor 108 (M1) and turning ontransistor 110 (M2) and transistor 112 (M3). This provides a dischargepath to nodes B and D while isolating Vb from Vpp.

In normal operation, 108 (M1) is on and 110 (M2) and 112 (M3) are off.If Vpp decreases from its nominal value, Vb also tends to decrease whichcauses the enable signal to go to an active high state thereby allowingmore clock pulses to pass through the AND gate 116 to the charge pump102. Due to the increase of clock pulses to the charge pump 102, Vpp isincreased thereby compensating for its initial decrease. Vpp isregulated at a value given by the following equations:Vpp=Vb*(1+C 2/C 1)orVpp=Vref*(1+C 2/C 1)

Both equations are easily derived by the definition of the dividedvoltage Vb and by the consideration that the input voltages of thedifferential amplifier are substantially equal.

More specifically in normal operation, even without and load applied toVpp, leakage associated with node B tends to discharge node B to groundthereby lowering Vb below Vref. As a consequence, the differentialamplifier 114 turns on, the enable signal goes active high, clock pulsesare coupled to the charge pump 102 and Vpp increases to compensate forthe leakage loss of charge on node B. To insure proper operation of thevoltage divider 118, a short reset pulse is generated by a circuit (notshown) on the rising edge of the enable signal, as illustrated in FIG.2. This short reset pulse resets the capacitive voltage divider 118.Grounding node B during the reset does not harm the operation, since theenable signal is already high. As soon as the reset signal ends, Vb goesto its nominal divided value and the differential amplifier 114 startssensing the voltage at node B.

Capacitors 104 (C1) and 106 (C2) of the voltage divider 118 of FIG. 1,are sized taking into account the ratio C2/C1, which gives the desiredVpp. They cannot be sized too big or an excessive capacitive load willbe placed on the charge pump 102, likewise, they cannot be sized toosmall or they may discharge to quickly. In order to integrate capacitors104 (C1) and 106 (C2) into a flash memory chip, designers typically useavailable components usually found in a typical flash memory technology.Otherwise, it would be very expensive to generate extra process stepsjust to build the capacitors needed for the voltage divider 118.Moreover, for space reasons, the capacitors must occupy the least amountof silicon area as possible. In modem flash technology, an amount ofsilicon area occupied by a typical capacitor is in the range of 200um2/pf.

Typically, a capacitor 300 in flash memory chip is built with a polysilicon area 302 positioned on top of a N− well region 304, asillustrated in FIG. 3. The N− well region 304 is formed in a P-substrate310 of the flash memory chip. As illustrate in FIG. 3, the poly 302 isthe first terminal T1 of the capacitor 300. A pair of n+ regions 306 and308 are formed in the N− well region 304 to form two contacts. Thesecontacts are the second terminal T2 of the capacitor 300. A thindielectric layer 312 is positioned between the poly 302 and the N− well304. A common type of dielectric layer 312 used is silicon oxide havingan approximate thickness of 30 Å. The “N’ and “n’ denote a N type ofdonor impurity and the “P” denotes a P type donor impurity. The “−”denotes a low donor impurity density and the “+” denote a high donorimpurity density. A schematic representation of T1 and T2 of capacitor300 is shown in FIG. 3A.

One problem with the capacitor illustrated in FIG. 3 is that it is notlinear. This is illustrated in the table of FIG. 4. In particular, as T1becomes positive verses T2, the N− well region 304 just below the poly302 becomes more and more populated by electrons. This reduces theelectric equivalent distance between the two terminals of the capacitorthereby increasing the capacitance. Moreover, as T2 becomes positiveversus T1, the N− well region 304 below the poly 302 starts to bedepleted of electrons. This may cause the capacitor to become invertedresulting in a decrease in capacitance.

In addition to the non-linearity, there typically is a spread of valuesof capacitance due to both the varying thickness of the dielectric layer312 and the spread of the doping in the N− well region 304 approximatethe surface of the dielectric layer 312. Since, the divided voltage Vbdepends from the value of the C2/C1 ratio, not by the value of a singlecapacitor, and since the voltage across C1 and C2 are generally not thesame, C1 and C2 may work at different point of their non-linier C-Vcharacteristics and suffer from the related uncontrollable spread ofcapacitance values. Another issue encountered in the capacitor 300 ofFIG. 3 is parasitic capacitance Cpar 314 that occurs between the N− well304 and the P-substrate 310. In building a voltage divider, T1 mustalways be more positive than T2. Otherwise the capacitance becomes tolow due to the non-linearity. Hence, when putting two capacitors inseries, the intermediate node (node B in FIG. 1) is affected by Cpar314.

The non-linearity, the spread and the additional leakage, all contributeto lower the precision of the regulated charge pump. The problems allstem from the variability and the instability of the distance d in theclassic formula of the capacitor, C=εS/d. Where ε is the dielectricconstant of the insulator, S is the surface area of the insulatorbetween the 2 terminals and d is the distance between the two terminals.While the location of terminal T1 in the poly 302 in FIG. 3 is fixed,the position of the N− wells 306 and 308 that form terminal T2 may vary,hence the distance d may vary. Therefore, the capacitance of FIG. 3 mayvary because of the inherent nature of the semiconductor design.

There are other means to integrate a capacitor into an integrated flashmemory to avoid the aforementioned problems. For example, a metal overpoly capacitor or a first metal over a second metal capacitor could beused. However, the distance d in these examples is almost two orders ofmagnitude higher. The most convenient method to integrate a capacitormay be to use an inter-level silicon oxide sandwiched between a firstmetal and a second metal layer. However, such a capacitor would occupyan area of a range of 13,000 um2/pf. That is 65 times the area occupiedby the poly/gate oxide/N− well capacitor of FIG. 3. Moreover, in thisdesign stray capacitance is somewhat difficult to control therebyaffecting the precision of the divider. A more effective means forintegrating a capacitor into an integrated flash memory would be to usea stack structure of a poly silicon layer, a first metal layer and asecond metal layer. Although, with this design the stray capacitancecould be minimized by connecting the first metal of C1 with the firstmetal of C2 in a divider circuit, the silicon area of such a structurewould be about 8000 um2/pf, which is 40 times that of the poly/gateoxide/N− well capacitor of FIG. 3.

In modem integrated circuits, including integrated flash memorycircuits, the distance between two adjacent lines or layers of metalsused to form the integrate circuits have been scaled down. As a result,the parasitic capacitance between the adjacent metal lines or layers hasincreased. The present invention takes advantage of this by using theparasitic capacitance to build a voltage divider.

Referring to FIG. 5, a cross-sectional top view of one embodiment of acapacitor 500 of the present invention is illustrated. As illustrated,the capacitor 500 is made in the layers of material used to make theintegrated circuit. In particular, the capacitor 500 is made in a polysilicon layer 502, a first metal layer 504 and a second metal layer 506.The first and second metal layers 504 and 506 are made of conductingmetals including, but limited to, copper, aluminum, gold and silver.Moreover, FIG. 5 specifically illustrates the cross-sectional top viewof the first metal layer 504. As illustrated, the first metal layer 504is fabricated to form a first terminal 510 (T1) and a second terminal520 (T2). The first terminal 510 (T1) is electrically isolated from thesecond terminal 520 (T2).

The first terminal 510 (T1) is formed into a first base strip 512 and aplurality of first side strips 514. The first side strips 514 extendfrom one side of the first base strip 512. In one embodiment, the firstside strips 514 extend generally perpendicular from the first base strip512. The second terminal 520 (T2) is formed into a second base strip 522and a plurality of second side strips 524. The second side strips 524extend from the second base strip 522. In one embodiment, the secondside strips 524 extend generally perpendicular from the second basestrip 522. The first side strips 514 of the first terminal 510 (T1) arepositioned in between the second side strips 524 of the second terminal520 (T2) so that the first and second side strips 514 and 524 arealternately positioned between the first base strip 512 and the secondbase strip 522. The poly silicon layer 502 is coupled to the secondterminal 520 (T2) with a plurality of poly-T2 contacts 530 (firstcontacts 530). In addition, the second metal layer 506 is also coupledto the second terminal 520 (T2) by a plurality of second metal-T2contacts 532 (second contacts 532). In one embodiment, the poly-T2contacts 530 and the second metal-T2 contacts 532 are coupled to thesecond base strip 522 of the second terminal 520 (T2).

A cross-sectional side view of one embodiment of the capacitor 500 ofthe present invention is illustrated in FIG. 5A. As illustrated, thefirst metal layer 504 is positioned between the poly silicon layer 502and the second metal layer 506. The capacitance of capacitor 500 is thesum of the parasitic capacitance as illustrated by 540(Ca), 542(Cb),544(Cc) and 546(Cd) of FIG. 5A. The silicon area of capacitor 500 in anintegrated flash memory circuit is about 6000 um2/pf. Although, this isabout 30 times the area of the poly/gate oxide/N− well capacitor of FIG.3, it is less than 50% the size of a conventional first metal/siliconoxide/second metal capacitor. In addition, the capacitor of the presentinvention is about 25% the size of a stacked structure of polysilicon/first metal/second metal. Moreover, capacitor 500 of the presentinvention is a relatively stable and precise capacitor. In addition, asintegrated circuits become more and more scaled down, the distancebetween side strips 514 and 524 will be decreased. This will cause thecapacitance of 542 (Cb) and 544 (Cc) to become more predominate therebymaking the advantages of the present invention more evident.

A schematic diagram of a charge pump circuit 600 of one embodiment ofthe present invention is illustrated in FIG. 6. When coupling twocapacitors 500 of the present invention to create a voltage divider 602,the first terminal 510 (T1) of a first capacitor 500 (C1) is coupled tothe first terminal 510 (T1) of a second capacitor 500 (C2). The straycapacitance of capacitor 500 (C2) associated with the second terminal520 (T2) is coupled to ground as shown. Moreover, stray capacitanceassociated with the second terminal 520 (T2) of capacitor 500 (C1) iscoupled to Vpp via transistor 108 (M1).

Referring to FIG. 7, a simplified block diagram of an integrated flashmemory device 700 of one embodiment of the present invention isillustrated. As illustrated, the flash memory device 700 includes thecharge pump circuit 600 of the present invention to provide voltages ofa predetermined level to memory cells within the memory array 710 duringmemory operations. Control circuitry 704 is provided to control memoryoperations to the memory array 710. An address register 706 is used toreceive address requests to memory array 710. In addition, aninput/output (I/O) buffer 708 is used to smooth out the flow of data toand from the memory array 710. FIG. 7 also illustrates an exteriorprocessor 750. Processor 750 is coupled to the control circuitry 704 tosupply control commands. Processor 750 is also coupled to the addressregister to supply address requests. Moreover, processor 750 is coupledto the I/O buffer 708 to send and receive data.

CONCLUSION

A capacitor comprising a poly silicon layer, a first metal layer and asecond metal layer has been disclosed. In one embodiment, the firstmetal layer is positioned between the poly silicon layer and the secondmetal layer. The first metal layer has a first terminal and a secondterminal. The first terminal is electrically isolated from the secondterminal.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

1. A voltage divider comprising: a first capacitor having a secondterminal selectively coupled to a voltage supply; a second capacitorhaving a first terminal coupled to a first terminal of the firstcapacitor, the second capacitor further having a second terminal coupledto ground; wherein the first and second capacitor include: a first metallayer forming the first and second terminals, the first and secondterminals each having a base strip and a plurality of side strips thatextend from the respective base strip, wherein the side strips of thefirst terminal are positioned in between the side strips of the secondterminal so that the first and second side strips are alternatelypositioned between the first and second bases of the respective firstand second terminals, further wherein the first terminal is electricallyisolated from the second terminal; a second metal layer; and a polysilicon layer, the first metal layer is positioned between the polysilicon layer and the second metal layer and the first terminal of thefirst metal layer is electrically isolated from the poly silicon layer.2. The voltage divider of claim 1, further comprising: a plurality offirst contacts to couple the poly silicon layer to the second terminalof the first metal layer of the first and second capacitors; and aplurality of second contacts to couple the second metal layer to thesecond terminal of the first metal layer of the first and secondcapacitors.
 3. The voltage divider of claim 1, wherein the plurality offirst side strips extend generally perpendicular from the first basestrip and the plurality of second side strips extend generallyperpendicular from the second base strip.
 4. A voltage dividercomprising: a first capacitor having a second terminal selectivelycoupled to receive a voltage supply; a second capacitor having a firstterminal coupled to a first terminal of the first capacitor, the secondcapacitor further having a second terminal coupled to receive a ground;wherein the first and second capacitor include: a poly silicon layer; asecond metal layer; and a first metal layer positioned between the polysilicon layer and the second metal layer, the first metal layer havingthe first and second terminals, wherein the first terminal iselectrically isolated from the second terminal and the poly siliconlayer.
 5. The voltage divider of claim 4, wherein the poly silicon layerof each first and second capacitor is coupled to the second terminal ofthe first metal layer.
 6. The voltage divider of claim 5, wherein thesecond metal layer of each first and second capacitor is also coupled tothe second terminal of the first metal layer in each capacitor.
 7. Avoltage divider comprising: a first capacitor having a second terminalselectively coupled to receive a voltage supply; a second capacitorhaving a first terminal coupled to a first terminal of the firstcapacitor, the second capacitor further having a second terminal coupledto receive a ground; wherein the first and second capacitor include: apoly silicon layer; the first metal terminal formed over the polysilicon layer and having a first base strip and a plurality of firstside strips extending from the first base strip, the first metalterminal electrically isolated from the poly silicon layer; the secondmetal terminal formed over the poly silicon layer and having a secondbase strip and a plurality of second side strips extending from thesecond base strip, the second metal terminal electrically isolated fromthe first metal terminal and electrically coupled to the poly siliconlayer; and a metal layer formed over the first and second metalterminals and electrically coupled to the second metal terminal.
 8. Thevoltage divider of claim 7, wherein the plurality of first side stripsextend generally perpendicular from the first base strip and theplurality of second side strips extend generally perpendicular from thesecond base strip.